window.onload = function () {
	
			// 请求动画帧
			window.requestAnimationFrame = window.requestAnimationFrame ||
			window.mozRequestAnimationFrame ||
			window.webkitRequestAnimationFrame ||
			window.msRequestAnimationFrame;
			
			var ceil1 = document.getElementById('ceil1');
			var ceil2 = document.getElementById('ceil2');
			var start = null;
			
			// startTimes 是一个时间参数,单位是毫秒
			// 表示当前时间距离requestAnimationFrame回调的时间
			function step (startTimes) {
				// console.log(startTimes);
				if (start == null) {
					start = startTimes;
				};
				
				var progress = startTimes - start;
				// console.log(progress / 10);
				
				ceil1.style.left =  - progress / 5 + 'px';
				ceil2.style.left = 495 + progress / 5 + 'px';
				if (startTimes < 2490) {
					requestAnimationFrame(step);
				};
				
			};
			
			requestAnimationFrame(step);
};
			
